ECE544 Mid-Term Page 1 Spring 2002 D. Raychaudhuri ECE 544 Computer Networks II Mid-Term Exam March 29, 2002 Profs. D. Raychaudhuri & M. Ott Instructions: This is a 2 hr, OPEN BOOK exam. (Only the textbook, Peterson & Davie, “Computer Networks, A Systems Approach” and 2 pages of notes are permitted.) Answer Q1 (40 points) and ANY THREE from Q 2-6 (20 points for each question). If you answer more than 3, your grade will be determined as Q1 + the best 3 from Q2-6. In addition to the correct final answer, credit will be given for logical development of each problem using network architecture diagrams, protocol definitions, timing diagrams, flow charts, pseudo-code, etc. where appropriate. Please be sure to state your assumptions, give concise explanations (no essays!), organize your work logically, and write with legible handwriting. Question 1: Network Architecture: A company with offices in three locations wishes to establish an IP-based broadband services data network with the following requirements: 1. Geographic locations: Atlanta, GA; Princeton, NJ and Palo Alto, CA. 2. ~100 employees at each location, each with a networked PC; 25% of these users have an additional laptop computer with wireless access. 3. A file server and compute server at each site. 4. Primary applications for PC users: email, www, ftp, mbone, NetMeeting (an IP-based multiparty teleconference application). 5. Estimated traffic per active user is 100 Kbps average and ~1 Mbps peak. Design an IP network that provides both local-area and wide-area connectivity for this company. Your answer should concentrate on Layer 2 & 3 functionality and must provide a complete view of components and protocols used. Use the following outline: 1. A high-level drawing of the whole network, identifying each major networking component (including Ethernet or wireless LAN, ARP servers, internal routers/ switches, DHCP servers, Internet backbone, etc.). [10 pts] 2. Specify the protocol stack(s) used at each network node in the system for (a) control, including addressing, routing, QoS management, etc.; and (b) for data transfer from application-to-application (for each application listed above) [5 pts] 3. Explain how end-users addresses (MAC address, IP address, etc.) are assigned, by what entities and using what protocols. [5 pts] 4. Each of the 3 locations can be considered an IP intra-domain. How is intra-domain routing done, by what entities and using what protocols? [5 pts] 5. What protocol is used for inter-domain routing (i.e. from one site to another) over the public Internet? Identify the network elements which use interdomain routing, explain how it works, and draw the routing protocol stack at each such node. [5 pts]ECE544 Mid-Term Page 2 Spring 2002 D. Raychaudhuri 6. Would you recommend any special features such as IP QoS or multicast for this network? If so, indicate what network elements are needed to support these features and show what changes are required in the protocol stacks drawn in item 2 above. [5 pts] 7. Estimate the link bandwidth required for each local site’s connection to the Internet, based on the ~100 users with traffic profiles specified above. [Make suitable assumptions for local vs. external traffic flows.] … [5 pts] Question 2. Shared Media LAN: 1. Explain the basic principle of Ethernet local area networks. Why is the length of an Ethernet segment limited to ~500m? What is the reason for exponential backoff after a collision occurs? Sketch the typical throughput vs. offered traffic curve for Ethernet. [5 pts] 2. Explain the basic principle of 802.11b WLAN. Why is collision avoidance used instead of collision detection? How does the RTS/CTS procedure help avoid hidden terminal problems? [5 pts] 3. Consider the Ethernet LAN with three users as shown in the figure below. In the above, assume that users U1, U2 and U3 transmit packets of fixed length 10T, where T is the end-to-end propagation delay. Assume for simplicity that packet arrivals and transmissions at terminals occur at virtual slot boundaries t=0,T,2T,3T,4T,…nT. Also, assume that the shortest Ethernet packet (after collision) is T sec, and that backoff algorithms delay by kT units of time, where k=1,2,3.... Now suppose U1, U2 and U3 wish to transmit a new packet at times t1=0, t2=0 and T3=5T respectively. Draw the sequence of transmissions and retransmissions on the channel from t=0 to the time when all three packets P1, P2, P3 have been transmitted successfully (assuming no other traffic on the LAN). Provide explanations for each channel event. Use the following tabular format to give your results, and explain each entry with numbered footnotes. …[10 pts] Time T1 activity or state T2 activity or state T3 activity or state 0 Packet arrives and is transmitted Packet arrives and is transmitted Idle T 2T 3T Shared Media (Ethernet) U1 U2 U3 Prop del = T/2 Prop del = T/2ECE544 Mid-Term Page 3 Spring 2002 D. Raychaudhuri Question 3. ATM Network: Consider the ATM network shown below. The terminal TA connects to a server SA to download a video-on-demand movie. The application program at the TA first retrieves a program guide via TCP/AAL5 over a UBR VC, and then after the TA selects a movie, the SA sends an MPEG2 movie via UDP/AAL5 using a VBR VC. 1. Show the sequence of signaling events at each node in the network necessary to set up the path for (a) the program guide query, and (b) video delivery. Present your results in the form of a protocol timing diagram at each node, using the format shown in the figure. [5 pts] 2. Fill in the corresponding VPI/VCI forwarding table entries at each ATM adapter and switch in the end-to-end connection. Use port numbers as indicated in the figure and assign VCI numbers arbitrarily in the range 128-256; set VPI=0. [5 pts] 3. Show how the AAL segments the first TCP query message into the ATM cell format used for data transfer. [5pts] 4. Explain how quality-of-service (QoS) is set up for the video connection. Describe applicable traffic shaping, call admission control and scheduling functions. [5 pts] Format of VCI/VPI forwarding table at each switch: Input Port # VPI/VCI Output Port # VPI/VCI 2 0.137 4 0.211 …. …. …. …. TA SA ATM Adaptor (TA) ATM Adaptor (SA) ATM Switch A ATM Switch B ATM Switch C Port 1 Port